A college football player who has been diagnosed with a concussion is likely to have a smaller hippocampus, the memory center of the brain, than a player who hasn’t been so diagnosed, a new study finds. And regardless of whether they’d had concussions, players have smaller hippocampi than men their age who don’t play football and who have no history of brain trauma, the study suggests.

“This is one of the first papers to draw a direct link from concussion to specific tissue changes,” says Dennis Molfese, a neuropsychologist at the University of Nebraska–Lincoln, calling the results intriguing.

Sports-related trauma studies have focused on the hippocampus because some memory deficits are linked to head injuries. But much of that work has investigated people who were middle-aged or older, says Patrick Bellgowan, an experimental psychologist at the Laureate Institute for Brain Research in Tulsa, Okla., and the University of Tulsa.

He and his colleagues measured hippocampus size of 50 Division I college football players and 25 male volunteers of similar age who don’t play football or soccer. MRI scans revealed that 25 players who had had a concussion had hippocampi that averaged just three-quarters the size of those of men who hadn’t played football or soccer. The same brain area in the 25 football players who hadn’t suffered a concussion was about five-sixths as large as in the control group, the researchers report in the May 14 JAMA.

The researchers also found that having more years of football experience was associated with slower reaction time on tests.

Bellgowan says the difference in hippocampus volume between the nonplayers and the football players who hadn’t had a concussion suggests cumulative damage to the hippocampus could come from injuries that fall short of diagnosed concussions. “It was a bit of a surprise to find that both groups [of players] were smaller,” he says.

The mechanisms underlying this volume loss are unknown. An inflammatory reaction might cause cells in the hippocampus to rev into an excited state and eventually die. “That’s the operating thesis,” Bellgowan says, but he has no direct evidence of it yet. A contact sport that damages the blood-brain barrier and triggers such inflammation might disrupt normal hippocampal processes, he says.

In this study, damage seemed limited to the hippocampus. The amygdala, which is near the hippocampus and shares some emotion and memory chores with it, “showed absolutely no difference between groups,” Bellgowan says.